Support => within case, and improve error messages for cond and case

Support => within case, and improve error messages for cond and case

* module/ice-9/boot-9.scm (cond, case): Reimplement using syntax-case,
  with improved error messages and support for '=>' within 'case' as
  mandated by the R7RS.  Add warnings for duplicate case datums and
  case datums that cannot be meaningfully compared using 'eqv?'.

* module/system/base/message.scm (%warning-types): Add 'bad-case-datum'
  and 'duplicate-case-datum' warning types.

* test-suite/tests/syntax.test (cond, case): Update tests to reflect
  improved error reporting.  Add tests for '=>' within 'case'.

* test-suite/tests/tree-il.test (partial evaluation): Update tests to
  reflect changes in how 'case' is expanded.

* doc/ref/api-control.texi (Conditionals): Document '=>' within 'case'.